The present disclosure relates generally to control circuit and method for driving light emitting diodes (LEDs), and more particularly to the control of dimming LEDs.
LEDs have obtained popularity in the field of lighting and backlight modules, due to their excellent lighting efficiency, compact product size, and long lifespan. For instance, the backlight modules used in computer monitors or televisions have largely turned to employ LEDs as their light sources, instead of cold cathode fluorescent lamps that were commonly used years ago.
FIG. 1 demonstrates a LED driver 10 in the art, which drives four LED strings S1˜S4 in parallel. Even though each LED strings in FIG. 1 has LEDs connected in series, one LED string in other example might have only one LED. A booster 12 boosts up an input power voltage VIN and generates a driving power voltage VOUT to a common power node of the LED strings S1˜S4. Current control circuits CD1˜CD4 govern the driving current I1LED˜I4LED through LED strings S1˜S4 respectively. LED controller 14 controls the operation of the current control circuit CD1˜CD4 and the booster 12.
FIG. 2 shows a conventional LED controller 14 in the art. A minimum voltage selector 20 provides a minimum feedback voltage VFBMIN based on the minimum of the feedback voltages V1FB˜V4FB, which are the voltages at feedback nodes FB1˜FB4 of LED strings S1˜S4. Transconductor 22 drives a compensation node COM based on the difference between the minimum feedback voltage VFBMIN and a reference voltage VREF, so as to charge or discharge a compensation capacitor 23 and to build a compensation voltage VCOM. A pulse-width modulator 24, in response to the compensation voltage VCOM, generates a PWM signal SDRV, which turns ON and OFF the power switch 28 periodically. Simply speaking, minimum voltage selector 20, transconductor 22, pulse-width modulator 24, booster 12, and LED strings S1˜S4 as a whole forms a loop with a negative loop gain, capable of stabilizing the minimum feedback voltage VFBMIN at the reference voltage VREF.
Constant current driver CC1˜CC4 correspond to current control circuits CD1˜CD4 respectively. Only the constant current driver CC1 is detailed herein because other constant current drivers are analogous in view of the teaching of the constant current driver CC1. The constant current driver CC1 has an operational amplifier 30, which is configured to make a current sense voltage V1CS about the same with the setting voltage VCSSET, which, depending on the dimming signal SDIM, is either 0V or a predetermined voltage VCSON. As the current sense voltage V1CS, in a way, represents the driving current I1LED, the constant current driver CC1 can stabilize the driving current I1LED.
The dimming signal SDIM is capable of adjusting the brightness of the LED strings S1˜S4, or dimming the LED strings S1˜S4. The dimming signal SDIM is a PWM signal, for example. When the dimming signal SDIM is “1” in logic, the minimum feedback voltage VFBMIN could be stabilized to be about the reference voltage VREF and each of driving currents I1LED˜I4LED is about a constant corresponding to the predetermined voltage VCSON, such that LED strings S1˜S4 emit light continuously and stably. In the opposite, when the dimming signal SDIM is “0” in logic, LED controller 14 constantly turns OFF the power switch 28 in the booster 12, and all the driving currents I1LED˜I4LED are to be 0 A, such that LED strings S1˜S4 do not emit light. This kind of dimming control is generally called PWM dimming. Here in this specification, a dimming ON period TDIM-ON refers to the period of time when the dimming signal SDIM is “1”, and a dimming OFF period TDIM-OFF to the period of time when the dimming signal SDIM is “0”. A dimming duty cycle, the ratio of one dimming ON period TDIM-ON to one cycle time of the dimming signal SDIM, is a factor substantially corresponding to the brightness of the LED strings S1˜S4. Dimming linearity refers to the correlation between the brightness of a light source and the dimming duty cycle. Perfect dimming linearity means the brightness of a light source is entirely proportional to the dimming duty cycle, and is always a dream that designers of lighting apparatuses or lighting controllers desire to achieve.